1. Field of the Invention
The invention relates to nanowires, and more particularly to the implementation of a varactor diode using nanowires and methods of making the same.
2. Background Art
A varactor is a semiconductor diode that functions like a voltage-dependent capacitor. The varactor diode comprises two semiconductor regions: a p-conductivity type doped region and an n-conductivity type doped region. One end of each region is joined to the other region at what is called a PN junction. The other end of each region has an ohmic contact, which allows the diode to be biased.
When no bias is applied, the different energy levels in the two regions cause a diffusion of the holes and electrons through both materials, which balances their energy levels. In the quiescent state (i.e., when this diffusion process stops), the diode is left with a small region on either side of the PN junction, called the depletion region, which contains no free electrons or holes. The movement of electrons through the materials creates an electric field across the depletion region that is described as a barrier potential and has the electrical characteristics of a charged capacitor.
Application of an external bias, applied in either the forward or reverse direction, affects the magnitude, barrier potential, and width of the depletion region. Enough forward bias will overcome the barrier potential and cause current to flow through the diode. Keeping the bias voltage at levels that do not allow current flow will permit the width of the depletion region to be controlled. Since the depletion region acts as a capacitor, the diode will perform as a variable capacitor that changes with the applied bias voltage.
Varactor diodes are used in various circuit applications, such as electronic tuning of oscillators, filters, RF and microwave multipliers, and the like. A unique characteristic of the variable capacitance property of the varactor lends to use in other circuit applications, such as amplifiers, to produce much lower internal noise levels than circuits that depend upon resistance properties. Since noise is of primary concern in receivers, for example, circuits using varactors are an important development in the field of low-noise amplification. Uses of varactors to date have been as the basic component in parametric amplifiers, phase-shifters, and the like.
With the continued miniaturization of circuit components, designers will inevitably seek components of nano dimensions. Nanowire field effect transistors (FETs) have been proposed, by Charles M. Lieber et al., in Published U.S. Patent Application 2002117659. Thus, what is needed is a nano-scale varactor diode.